Exemplary embodiments pertain to the art of actuators, more specifically jam-tolerant electro-mechanical linear actuator for application in aircraft.
Linear electromechanical actuators have been incorporated in aircraft over past years to operate critical flight elements, such as flight control surfaces and landing gear, while reducing fuel consumption due to the weight reduction obtained through the substitution of hydraulic and pneumatic systems for these lighter systems. Conventional linear electromechanical actuators with rotary induction motors have commonly been employed for flap and slat control of aircraft wing surfaces. Although, these linear electromechanical actuators can provide a convenient method of control, their lower force density, because the weight and volume, limits their applicability and scalability in airborne applications.
Conventional direct-drive linear permanent magnet (PM) motors produce force densities that are adequate for actuating various parts/loads onboard aircraft. On the other hand, modern linear actuators with ball screws or roller screws and rotary brushless DC PM motors produce much higher force density than conventional motors. Thus, for aircraft/engine architectures, ball screw or roller screw linear actuators with rotary PM brushless motors may prove advantageous because they can develop much higher thrust/torque for the same mass and volume envelope of conventional configuration. Moreover the greater force densities facilitate lower weight, and envelope capabilities. Potential applications for linear actuators in aircraft technology include, but are not limited to; flight control (both primary and secondary) surfaces; fuel systems management; lubrication systems management; aircraft equipment and environmental control systems.
Critical flight elements typically mount redundant linear electromechanical actuators to ensure their operability upon electrical or mechanical failure of one of the linear electromechanical actuators. To this end, the failed linear electromechanical actuator must freely extend and follow the movement of the working linear electromechanical actuator that continues to operate the critical flight element. Different solutions aimed at preventing failure of critical flight elements upon electrical or mechanical failure of one of the linear electromechanical actuators have been developed. A first solution consists of a linear electromechanical actuator with a screw-nut assembly engaged by means of a clutch to a gearbox driven by an electrical motor. Upon electrical or mechanical failure of the linear electromechanical actuator, actuation of the clutch disengages the screw-nut assembly from the gearbox, thus allowing free extension of the linear electromechanical actuator. Unfortunately, this solution does not prevent screw jamming, the main mechanical cause of failure of linear electromechanical actuators, as the disengagement occurs upstream of the screw-nut assembly.
Another solution consists of a pyrotechnic linear electromechanical actuator with a screw-nut assembly driven by an electric motor and a fuse-type piston engaged to the screw-nut assembly by retaining elements. Upon electrical or mechanical failure of the linear electromechanical actuator, explosive loads adjacent to the retaining elements are activated to destroy the retaining elements, which, in turn allows free extension of the linear electromechanical actuator. Any kind of electrical or mechanical failure will result in permanent disengagement of the fuse-type piston as the retaining elements have been destroyed. Therefore, the linear electromechanical actuator must be entirely mounted anew on the flight element after electrical or mechanical failure, increasing diagnostic and repair costs. Moreover, such systems, because of their destructive nature, are difficult to conduct thorough functional tests before installation.